(Reuters) - U.S. drugs regulators on Friday approved the stroke preventer Xarelto from Bayer AG and Johnson & Johnson for people with a common heart rhythm disorder called atrial fibrillation.

However, the Food and Drug Administration added a restrictive "black box" warning to the drug, warning people against discontinuing use of Xarelto without consulting their doctors, as stopping abruptly may increase risk of stroke.

Xarelto, clinically known as rivaroxaban, had been approved for anti-clotting after knee and hip surgery and already has European approval for this use.

The once daily anti-clotting pill is now among several drugs angling to replace decades-old warfarin that helps prevent stroke in people with atrial fibrillation, a market estimated to be worth more than $10 billion.

The drug is the first once daily anticoagulant pill that doesn't require routine blood monitoring, according to J&J. For Bayer, it is the biggest pipeline drug.

More than two million Americans have atrial fibrillation, meaning irregular heartbeats that can cause blood to pool and increase their risk of blood clots and strokes. But many are unwilling to take warfarin, which requires regular blood tests, or cannot tolerate it.

Bayer has said it expects more than 2 billion euros ($2.7 billion) in peak annual sales from the product.

An FDA panel recommended Xarelto, but the reviewers called for the agency to delay its approval because it was not proven to be at least as effective as older treatments. FDA advisers were divided on whether Xarelto was as effective as warfarin.

Echoing some panelists' concerns, the U.S. consumer group Public Citizen said the drug's label should not be expanded because it may cause a rebound occurrence of strokes when the medication is discontinued. The group also said clinical trials of the drug were conducted in a manner that favored Xarelto.

Boehringer Ingelheim, a privately held German company, was the first to market a warfarin replacement drug after the FDA approved its anti-clotting pill Pradaxa in October 2010.

But the company has said the pill was linked to about 50 deaths from bleeding across the world since its market launch, and European regulators said last month that patients taking Pradaxa should have their kidneys checked.

Another drug chasing Xarelto to market is Eliquis by Bristol-Myers Squibb and Pfizer.